Electric vehicles typically utilize an inverter in the form of a switch-mode power supply to provide three phase operating power to the vehicle's electric drive motor. The inverter includes a number of power switching devices that can supply the high currents needed. The inverter is usually located in an environmentally sealed module that is commonly referred to as the power electronics bay (PEB). This module typically includes other electronic circuits, such as those used to run the vehicle's electronic power steering, climate control compressor motor, and traction control system.
In an effort to minimize the amount of electromagnetic interference (EMI) radiated from the inverter and other circuitry within the power electronics bay, the circuits themselves are enclosed together within a grounded metal chassis. This chassis normally includes a housing having feedthrough electrical connectors (for power, control, and data signals) as well as an inlet and outlet coolant manifold that permit liquid coolant to be circulated through the power electronics bay for cooling of the inverter's power switching devices. In a typical liquid-cooled inverter application, the power switching devices are mounted by their baseplates to a conductive metallic liquid-interface heat exchanger. The coolant manifold of the heat exchanger that leads into and out of the chassis is metallic and is attached to the chassis. Thus, there is no electrical isolation between the power switching device baseplates, the heat exchanger and chassis. Where thermal grease or some other non-electrically insulating thermal interface is used between the switching device baseplates and heat exchanger, the baseplates themselves will be electrically connected to both the coolant and chassis. This produces capacitive coupling between the power switching devices and the chassis that is typically about 12-100 pF per die cm.sup.2, which can result in as much as 190 pF or more of capacitance. This allows undesirably high currents to be injected into the chassis, resulting in unwanted radiated emissions. Isolation of the baseplates and heat exchanger can be achieved using an insulating thermal interface such as a gasket; however, this seriously degrades the thermal coupling between the baseplates and heat exchanger.
Accordingly, there exists a need for a power electronics liquid-cooled heat sink mounting assembly that maintains good thermal conduction from the power switching devices while reducing the radiated EMI due to currents flowing from the switching devices and into the chassis.